Sorption of niobium on colloidal silica and the effect of humic acid

Sorption of niobium on silica colloids was studied using radiotracer technique in the pH range of 2-11 in NaClO4 medium. Silica was characterized using X-ray diffraction, light scattering and surface area measurements. The point of zero charge for silica colloids was about pH 2. The sorption of niobium was >99 % in the pH range of 3-9 and was slightly lower (similar to 95 %) below pH 3 and above pH 9. The quantitative sorption in the pH range 3-9 could be explained by surface complexation model. The small reduction in sorption was attributed to the existence of small fraction of niobium as cationic species at pH <= 2 and anionic species above pH 9. Presence of humic acid was found to have negligible effects on the sorption characteristics.